Generation of Iminoxyl Radicals by Photoredox Catalysis Enables Oxidant-Free Hydroxygenation of β,γ-Unsaturated Oximes

Abstract

A visible-light photocatalytic generation of iminoxyl radicals has been accomplished under oxidant-free conditions. This approach offers a mild, atom-economical, and redox-neutral synthesis of 5-methylisoxazolines through radical hydroxygenation of β,γ-unsaturated oximes in the absence of an additional hydrogen source.

Key words

oximes - hydroxygenation - isoxazolines - photoredox reaction - iminoxyl radical

Publication History

Received: 12 October 2021

Accepted after revision: 02 November 2021

Accepted Manuscript online:
02 November 2021

Article published online:
16 November 2021

• References and Notes

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• 7 Isoxazolines 2a–r; General ProcedureA flame-dried 8 mL tube was charged with the appropriate oxime 1 (0.2 mmol), Na₂CO₃ (31.8 mg, 0.3 mmol), and fac-[Ir(ppy₃)] (2.6 mg, 0.004mmol) in anhyd CHCl₃ (2.5 mL) under Ar. The resulting mixture was degassed by bubbling with Ar and then stirred for 36 h at rt with irradiation by 460 nm blue LEDs. The mixture was extracted with EtOAc (3 × 15 mL), and the combined organic layers were washed with brine (10 mL), dried (Na₂SO₄), filtered, and concentrated in vacuo. When the solvent had been completely removed, the residue was purified by column chromatography (silica gel).5-Methyl-3-(4-tolyl)-4,5-dihydroisoxazole (2a)Brown solid; yield: 21.6 mg (65%); mp 41−42 °C; R_f = 0.38 (PE–EtOAc, 10:1). ¹H NMR (500 MHz, CDCl₃): δ = 7.71–7.59 (m, 2 H), 7.47–7.33 (m, 3 H), 4.94–4.82 (m, 1 H), 3.43 (dd, J = 16.3, 10.1 Hz, 1 H), 2.93 (dd, J = 16.3, 8.0 Hz, 1 H), 1.43 (d, J = 6.2 Hz, 3 H).
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